This invention relates to an antilock control method for vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the wheel surface condition and the amount of slip between the wheel and the road surface. This braking force increases as slip increases until a critical value of slip is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock control system seeks to operate wheel slip at or near the critical slip value.
An antilock control system achieves this objective by detecting an incipient lock condition. Upon detecting an incipient lock condition, the antilock control system releases pressure at the wheel brake to allow recovery from the incipient lock condition. Upon recovery, brake pressure is re-applied. Criteria used to indicate an incipient lock condition includes excessive wheel deceleration and/or excessive wheel slip.
One known antilock control system uses a motor driven pressure modulator in which a DC torque motor drives a piston in a cylinder whose volume is modulated to control the hydraulic brake pressure at the wheel brake. In this system, because of the relationship between motor current, motor torque and motor load represented by the hydraulic brake pressure on the head of the piston while brake pressure is being applied, the value of motor current is used as a representation of brake pressure and is controlled to provide control of the brake pressure.
In one such system, when an incipient wheel lock condition is sensed, the value of motor current at this time is stored as a representation of the brake pressure producing the maximum braking force coexisting with the critical slip between the wheel and the road surface and the motor current is controlled to quickly retract the piston to release brake pressure to allow recovery from the incipient wheel lock condition. When a recovery from the incipient wheel lock condition is sensed, the motor current is controlled to extend the piston to re-apply brake pressure. In re-applying the brake pressure, the pressure is quickly established substantially at the brake pressure producing the maximum braking force by quickly establishing the motor current at a significant faction of the motor current stored at the time an incipient wheel lock condition was sensed. Thereafter, brake pressure is ramped at a controlled rate which may be a function of wheel slip and acceleration by ramping the motor current in direction applying brake pressure until an incipient wheel lock condition is again sensed after which the cycle is repeated.
In the pressure release phase of this antilock braking cycle, it is desirable to rapidly reduce brake pressure when an incipient wheel lock condition is sensed in order to prevent the wheel from locking. This may be provided in the motor driven modulator based wheel lock control system, as set forth above by applying maximum current through the motor in the direction causing retraction of the piston. However, to maximize braking efficiency, it is desirable to prevent an excessive decrease in the brake pressure as the pressure is reduced. To maximize the braking efficiency, the brake pressure should be reduced only enough to effect wheel recovery from the incipient wheel lock condition and return the wheel to the stable braking region. Accordingly, during the release phase of the antilock braking cycle, it is desirable to quickly "step-down" the brake pressure to a value that will effect recovery from the incipient wheel lock condition, but not to some lower pressure value.
In one known method of controlling the motor current to establish the step-down release of brake pressure when an incipient wheel lock condition is sensed, the motor current is controlled to quickly reduce the brake pressure when an incipient wheel lock condition is first sensed until the wheel parameters indicate the wheel is beginning to recover from the incipient wheel lock condition. The motor current is then controlled to maintain the brake pressure that existed at the time the wheel began recovering from the incipient wheel lock condition. However, in this form of establishing the step-down pressure, waiting for the wheel parameters to indicate the wheel beginning to recover from the incipient wheel lock condition while the motor is being controlled at a high speed to rapidly reduce pressure may result in an over-release of the brake pressure thereby decreasing the braking deficiency. Additionally, as a result of the high motor speed and system inertia, a further pressure drop will occur subsequent to the detection of the recovery condition thereby adding to the potential over-release in brake pressure.